1216 Chapter 32
For equipment having a grounding (three-conductor)
power cord, UL listing requires that its leakage current
be no more than 5 mA. Normally, this current would
flow through the safety ground path back to neutral and
would not trip a GFCI that has an intact safety ground
connection. However, if the safety ground is lifted and
the equipment is connected to other system equipment
via signal cables, the leakage current will flow in these
cables to reach ground, and ultimately neutral. Because
the current is not returning via the equipment’s own
power cord, the GFCI considers it hazardous and may
trip, since 5 mA is within its trip range. If multiple
pieces of equipment are plugged into a single
GFCI-protected circuit, the cumulative leakage currents
can easily become high enough to trip the GFCI. This
problem severely limits the ability of the GFCI/
ground-lift combo to solve ground loop prob-
lems—even when balanced power partially cancels
leakage currents.
32.7.3 Surge Protection
Haphazard placement of common surge protectors can
actually result in damage to interface hardware if the
devices are powered from different branch circuits.^56 As
shown in Fig. 32-61, very high voltages can occur
should there be an actual surge. The example shows a
common protective device using three metal-oxide varis-
tors, usually called MOVs, which limit voltage to about
600 Vpeak under very high-current surge conditions.
For protection against lightning-induced power line
surges, this author strongly recommends that MOV
protective devices, if used at all, be installed only at the
main service entry. At subpanels or on branch circuits to
protect individual groups of equipment, use series-mode
suppressors, such as those by Surge-X, that do not dump
surge energy into the safety ground system, creating
noise and dangerous voltage differences.57,5832.7.4 Exotic Audio CablesIn the broadest general meaning of the word, every
cable is a transmission line. However, the behavior of
audio cables less than a few thousand feet long can be
fully and rigorously described without transmission line
theory. But this theory is often used as a starting point
for pseudotechnical arguments that defy all known laws
of physics and culminate in outrageous performance
claims for audio cables. By some estimates, these
specialty cables are now about a $200 million per year
business.
Beware of cable mysticism! There is nothing unex-
plainable about audible differences among cables. For
example, it is well known that the physical design of an
unbalanced cable affects common-impedance coupling
at ultrasonic and radio frequencies. Even very low
levels of this interference can cause audible spectral
contamination in downstream amplifiers.^59 Of course,
the real solution is to prevent common-impedance
coupling in the first place with a ground isolator, instead
of agonizing over which exotic cable makes the most
pleasing subtle improvement. Expensive and exotic
cables, even if double or triple shielded, made of 100%
pure unobtainium, and hand braided by Peruvian
virgins, will have NO significant effect on hum and
buzz problems! As discussed in Section 32.5.4,
shielding is usually a trivial issue compared to
common-impedance coupling in unbalanced interfaces.
It’s interesting to note that some designer cables selling
for $500/meter pair have no overall shield at
all—ground and signal wires are simply woven
together.Figure 32-60. Common scenario to produce nuisance trips of GFCI in power conditioner.Leakage current flow in serial cables High-power equipmentEquipment Equipment
Power
xfrm*Limit for UL listed equipmentGround liftedDisconnectedEntry GndCATVChassisb5 mA*GFCI outlet